Basically, the OP is up against the hw limitation of his camera that lacks HSS flash.

Even if you have a Nikon/Canon DSLR, the result would be the same without a flash that supports HSS.

The best bet is probably a 3-stop ND filter, with +2EV on the exposure compensation dial (assuming he is using matrix metering) and put the camera in DR400. The ND filter is needed to enable the camera to use DR400, as ISO is now 800. Let the camera decide on the aperture and shutter speed.

HSS sync would have helped in getting the OP's first image to work since it would have allowed the camera to sync with the flash. If I remember correctly, the sync speed on the XE1 is 1/180, so at 1/250 his camera never caught the flash even though it did fire.

That being said, his settings were not maxed out, so he could have still nailed the shot without HSS. His settings are 1/250, ISO200, f18, and the image was a little underexposed, so there's some wiggle room there. If he brought his shutter speed down to 1/180, his camera would have caught the flash and his image would have been closer to correct exposure. That may have solved his problem right there. If his flash was still not bright enough, taking a step or two closer to his subjects would likely take care it. If he wanted to keep the same exposure, I think his lens goes to f22.